Show students what packages are available in the test runner

The test runner only has a limited set of packages, since it is forbidden from having internet access. If a student submits a package.yaml with a package that is not in that set, there will be an error. To give students a better idea of what packages the test runner has, instead of forcing them to resort to trial and error, the track documentation shown to the students should allow them to discover this. Suggestion for how to do this:

Add an explanation to https://github.com/exercism/haskell/blob/main/docs/TESTS.md#using-packages, containing the following points (which you can paraphrase or refine as necessary):

* The available packages are those that come with GHC, listed in https://downloads.haskell.org/~ghc/9.0.1/docs/html/libraries/index.html, plus those that were added to the test runner, listed in https://github.com/exercism/haskell-test-runner/blob/main/pre-compiled/package.yaml.
* If a package isn't on that list but you think it should be, send a pull request adding it to https://github.com/exercism/haskell-test-runner/blob/main/pre-compiled/package.yaml.